Automatic tire chains are permanently mounted near the drive tires and engage by turning a switch, then move into position to fling the pieces of chain under the tires automatically. Automatic chains were invented in 1941 in the United States[1] and Sweden in 1977.[2]

Snow chains, or tire chains, are devices fitted to the tires of vehicles to provide maximum traction when driving through snow and ice.

Snow chains attach to the drive wheels of a vehicle or special systems deploy chains which swing under the tires automatically. Although named after steel chain, snow chains may be made of other materials and in a variety of patterns and strengths. Chains are usually sold in pairs and must be purchased to match a particular tire size (tire diameter and tread width). Driving with chains reduces fuel efficiency, and can reduce the allowable speed of the automobile to approximately 50 km/h (30 mph), but increase traction and braking. Some regions require chains to be used under some weather conditions but some areas prohibit the use of chains which can deteriorate road surfaces.

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Snow chains were invented in 1904 by Harry D. Weed in Canastota, New York. Weed received U.S. Patent Number 768495 for his "Grip-Tread for Pneumatic Tires" on August 23, 1904.[3] Weed's great-grandson, James Weed, said that Harry got the idea of creating chains for tires when he saw drivers wrap rope, or even vines, around their tires to increase traction on muddy or snowy roads, which were the norm at the turn of the 20th century. He sought to make a traction device that was more durable and would work with snow as well as mud.[4]

In July 1935, the Canadian Auguste Trudeau obtained a patent for his tread and anti-skidding chain.[5]

In snowy conditions, transportation authorities may require that snow chains or other traction aids be installed on vehicles, or at least supplied for them. This can apply to all vehicles, or only those without other traction aids, such as four-wheel drive or special tires. Local requirements may be enforced at checkpoints or by other type of inspection. Snow chains should be installed on one or more drive axles of the vehicle, with requirements varying for dual-tire or multi-driven-axle vehicles that range from 'one pair of tires on a driven axle' to 'all tires on all driven axles', possibly also one or both steering (front) wheels, requiring snow chains whenever required by signage or conditions.

Tires come with standardized tire code sizing information, found on the sidewalls of the tires. The first letter(s), indicate the vehicle type (P for passenger, LT for light truck). The next three digits indicate the tire's width in millimeters. The middle two digit number indicates the tire's height-to-width ratio. The next character is a letter 'R,' which indicates radial ply tires (rather than radius). followed by a final two digit number indicating the rim size for the vehicle's wheels.

Driving on dry roads with chains can cause a vehicle to slide when braking.

Not securing the chains tightly enough. Owners' manual of the chains recommends tightening a second time after driving a short distance and checking for tightness from time to time. If a chain comes loose, it should either be refastened or removed before it wraps around the drive axle of the vehicle.

Tensioners or adjusters may be required. (Some chains have automatic tensioners and may be damaged if tensioners are not used.)

Cable chains on a car tire, with a relatively simple and easy-to-secure design

Cable chains on a bus tire

Tire chains are available in a variety of types that have different advantages of cost, ride smoothness, traction, durability, ease of installation, and recommended travel speed.

Materials include steel (in the form of links or cables), polyurethane, rubber, and fabric. Steel, link chains are also available in a variety of carbon steel and steel alloys and link shapes. Link shapes include standard, twisted, square, and reinforced.[6] The shape of the links changes the flexibility, grip, and strength of the chain. The links can also have added studs or V-bars for an even more aggressive traction. The use of alloy steel and hardened steel add durability. Traction cables (cable chains, snow cables) attach like chains but are made from cable rather than chain.

Chain patterns include the ladder, diagonal, or pattern types. Ladder type chains have cross chains perpendicular to the road and look like a ladder when carefully laid on the ground. Diagonal chains the cross chains are diagonal to the road. Pattern types form a "net" over the tire such as a diamond or multiple diamond pattern. Some industrial pattern types also include studded, metal rings to which the chains attach and thus are called ring chains.

Most tire chains are wrapped around the circumference of the tires and held in place with rim chains, which may be chain or cable, elastic or adjustable tensioners. Automatic chains do not wrap around the tire but swing under the tire from devices permanently mounted under the vehicle which deploy via an electronic solenoid activated in the cab. Spikes-Spider (registered trade-mark) mount onto the tires from one side.

Alternatives include studded tires which are snow tires with metal studs individually mounted into holes in the treads, and emergency traction devices which may be similar to tire chains but mount around the tire through openings in the rim and snow socks which are fabric rather than chain or cable.

Mud chains are similar to snow chains but for off-road, four-wheel drive applications, and generally they are larger than snow chains.

Wheel tracks are heavy duty assemblies similar to chains but with rigid cross links such as sometimes used on logging equipment.

Laws vary considerably regarding the legality of snow chain use. Some countries require them in certain snowy conditions or during certain months of the year, while other countries prohibit their use altogether to preserve the surface of the roads.[7]